Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J20/00—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof
  • B01J20/02—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof comprising inorganic material
  • B01J20/10—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof comprising inorganic material comprising silica or silicate
  • B01J20/16—Alumino-silicates
  • B01J20/18—Synthetic zeolitic molecular sieves
  • B01J20/186—Chemical treatments in view of modifying the properties of the sieve, e.g. increasing the stability or the activity, also decreasing the activity
• • C—CHEMISTRY; METALLURGY
  • C07—ORGANIC CHEMISTRY
  • C07C—ACYCLIC OR CARBOCYCLIC COMPOUNDS
  • C07C7/00—Purification; Separation; Use of additives
  • C07C7/12—Purification; Separation; Use of additives by adsorption, i.e. purification or separation of hydrocarbons with the aid of solids, e.g. with ion-exchangers
  • C07C7/13—Purification; Separation; Use of additives by adsorption, i.e. purification or separation of hydrocarbons with the aid of solids, e.g. with ion-exchangers by molecular-sieve technique

Definitions

• This invention relates to the separation of a xylene from admixture with at least one of its isomers and/or ethylbenzene.
• this invention relates to the separation of paraxylene from the various xylene isomers and/or ethylbenzene by contacting a mixture of paraxylene and at least one of the other xylene isomers and/or ethylbenzene with a predominantly potassium exchange form of type Y zeolite which has been modified so that the unit cell size of the zeolite is between 24.50 and 24.75 Angstroms, preferably between 24.60 and 24.70 Angstroms.
• the unit cell size of the zeolite may be defined as the smallest unit of structure which can show all the symmetrical properties of the zeolite crystal.
• the end result of modification can also be expressed in terms of exchangeable cation content, which is lowered by modification.
• the exchangeable sites i.e., the maximum attainable potassium content
• the exchangeable sites are reduced from approximately 0.50 mole per 100 g of volatile free zeolite to about 0.08 mole per 100 g and preferably to about 0.20 to 0.40 mole/100 g.
• the modified zeolite adsorbent of the instant invention is prepared from sodium Y zeolite, the usual form available commercially.
• the first critical step in the preparation is hydrothermal treatment of the zeolite with an ammonium salt solution (e.g. NH NO or NH Cl) to effect replacement of at least 70 mole percent and preferably 80 90 mole percent of the zeolites sodium ion content with ammonium ion.
• the ammonium Y zeolite is then thermally treated at 600 l,400 F., preferably between 800 and i,l F for periods of from one-half hour to 16 hours. Thermal treatment in the presence of water vapor or ammonia is essential if structural stability is to be maintained.
• the thermally treated zeolite is hydrothermally treated with a potassium salt solution (KCl, KNO etc.) to produce a predominately potassium exchanged zeolite.
• KCl, KNO etc. a potassium salt solution
• Paraxylene is a well known compound which is an intermediate in the manufacture of terephthalic acid.
• Terephthalic acid is subsequently employed in the manufacture of various synthetic fibers such as Dacron which fiber is a trade-marked product of the duPont Company.
• Dacron which fiber is a trade-marked product of the duPont Company.
• the ever increasing demand for such fibers has resulted in a corresponding increase in the demand for paraxylene.
• a xylene preferably paraxylene is preferentially adsorbed from admixture with at least one other aromatic when said mixture is contacted, under adsorption conditions, with a type Y zeolite containing predominantly potassium ions which has been altered (modified) so that its unit cell size is between about 24.50 and 24.75 Angstroms, preferably between about 24.60 and 24.70 Angstroms.
• the ordinary unit cell size for the predominately potassium exchanged form of a type Y molecular sieve is between about 24.76 and 24.85 Angstroms thus, a substantial difference in size is present.
• potassium ions By predominantly potassium ions it is meant the maximum concentration of potassium attainable by repetitive hydrothermal exchanges with a potassium salt solution. Depending on the severity of the thermal treatment prior to the hydrothermal exchange, this concentration can range from 0.08 mole/ gram of volatile free zeolite to approximately 0.40 mole/100 gram and preferably from 0.20 mole to 0.40 mole/100 g. Small residual concentrations of sodium cation 0.002 mole/100 g) are also present as well as a hydrogen ion content of undetermined level.
• the thermally treated ammonium-Y zeolite of the instant invention is to be hydrotherrnally exchanged with a potassium salt solution it is essential that the thermal treatment be carried out in the presence of water vapor.
• the hydrogen ion content of the steamed ammonium-Y zeolite is normally less than one-half that of the material obtained by thermal treatment in the absence of water.
• the steam treatment has resulted in a reduction in the cation content, a phenomenon thought to be directly related to loss of alumina from the zeolite structure. That the exchangeable cation content has been lowered by steaming is also apparent as evidenced from a comparison of the maximum potassium level attainable by hydrothermal exchange both before and after the steam treatment of the ammonium-Y zeolite.
• Adsorption may take place in either the liquid or vapor phase; however, if the liquid phase is utilized it is much preferred to pass an inert carrier liquid over the adsorbent bed in conjunction with or immediately following the feed steam.
• the adsorption of the paraxylene may take place at temperatures which vary broadly between 30 and 850 F, pressure may vary between 0.01 and 50 atmospheres.
• polar agents include water, simple alcohols, ketones, ethers, halogenated aliphatic hydrocarbons, naphthenes and aromatics other than C
• strongly polar agents alcohols, ethers, ammonia, ketones, etc.
• the modified adsorbent of the instant invention having been formed in part by high temperature treatment with steam is particularly stable toward steam and is thus particularly amenable to desorption with steam. Again, the operating temperature must be above about 250 F to insure facile reversal of steam desorption by treatment with fresh xylene feed.
• Paraxylene is usually found in admixture with orthoxylene, meta-xylene and ethylbenzene. It is within the scope of the instant invention to separate paraxylene from at least one of the previously mentioned constituents or, alternatively, any combination including all of them.
• the order of adsorptive ability of the predominantly potassium exchanged modified type Y sieve is paraxylene ethylbenzene meta-xylene ortho-xylene.
• the instant invention can by proper choice of feed stream and staging, be used to separate any one isomer from another. It should be emphasized that only a predominantly potassium exchanged form of the modified type Y zeolite may be used for the separations which are effected by the instant invention.
• the predominantly potassium exchanged form of the modified zeolite must exhibit a unit cell size between 24.50 and 24.75 Angstroms, preferably 24.60 and 24.70 Angstroms.
• the zeolite which is to be utilized as the adsorbent in the instant invention is made by exchanging 98 percent, then thermally treating in the presence of steam or ammonia, preferably steam, at a temperature in the range of 600 l,400 F for periods of from one-half hour to 16 hours, preferably at 800 l,l00 F for 1 to 4 hours and most preferably at 900 1,000 F for 1 to 2 hours, and finally exchanging residual sodium, ammonium and hydrogen cations with potassium to produce a predominantly potassium exchanged, modified Y zeolite.
• steam or ammonia preferably steam
• zeolite type Y molecular sieve In US. Pat. No. 3,130.007 there isdescribed the zeolite type Y molecular sieve; the disclosure of this patent is herein incorporated by reference.
• the crystals of zeolite Y are basically three-dimensional frameworks of SiO, and A10 tetrahedra cross-linked by the sharing of oxygen atoms.
• the valence of each tetrahedron containing aluminum is balanced by the presence in the alurninosilicate framework of a cation such as a sodium metal ion.
• the void spaces in the framework are occupied by water molecules. Dehydration to effect the lowest order of hydration results in a crystal interlaced with channels of molecular dimen-. sions which offer high selectivity and surface area for the adsorption of particular molecules.
• the basic chemical formula for zeolite Y expressed in terms of the metal oxides is shown below:
• n the valence of the cation M and X and Y represent the number of moles of Si0 and of water.
• the sodium form of the type Y molecular sieve is treated hydrotherrnally with an aqueous solution of an ammonium salt; chloride, nitrate or carbonate are preferred.
• the treatment involves agitation of the powdered type Y zeolite particles with the solution at temperatures from about ambient to about 180 to 220 F.
• An excess of ammonium ion is present above that required to replace the sodium ions ordinarily found in type Y zeolite stoichiometrically.
• a minimum of a 2- mole excess is preferred.
• the excess liquid is removed by filtration, decantation, or centrifugation.
• a second treatment is then carried out in the same manner, followed by a third and fourth if it is desired to obtain the maximum exchange of ions.
• a minimum of at least two treatments should be employed.
• the liquid is removed and the solid particles are washed free of excess salt.
• the zeolite is modified so that the unit cell size is reduced from about 24.72 Angstroms to about 24.44 A preferably to about 24.50 to 24.70 A
• the unit cell size is modified by thermal treatment in the presence of water vapor according to the general procedure specified by Eberly et al. in US. Pat. No. 3,506,400 herein incorporated by reference.
• the treatment consists of heating the ammonium-Y zeolite at an elevated temperature within the range of about 600 l,400 F in an atmosphere containing at least 2 percent water by weight for a period of time sufficient to effect removal of at least some alumina tetrahedra from the crystal lattice of the zeolite.
• the NH Y zeolite is heated at 900 F in an atmosphere of steam for a period of 1 to 2 hours.
• the thermally modified form of the type Y zeolite is then treated hydrothermally with an aqueous solution of a potassium salt; chloride, nitrate or carbonate are preferred.
• the treatment involves agitation of the powdered type Y zeolite particles with the solution at temperatures from about ambient to about 180 to 220 F.
• An excess of potassium ion is present above that required to replace the sodium ions ordinarily found in the unmodified type Y zeolite stoichiometrically.
• a minimum of a 2-mole excess is preferred.
• the excess liquid is removed by filtration, decantation, or centrifugation.
• a second treatment is then carried out in the same manner, followed by a third and fourth if it is desired to obtain the maximum exchange of ions.
• a minimum of at least two treatments should be employed.
• the liquid is removed and the solid particles are washed free of chloride ions, if potassium chloride is used as the exchange salt or source of potassium ions.
• the solid particles are then dried, and form into pellets, extrudates, or any other desired form for use. They are calcined at temperatures of up to about l,200 F., in air or nitrogen, pfrior to actual use in order to remove any excess water and to destroy any nitrate or carbonate ions which remain if these materials are used as exchange salts.
• thermally treated Y zeolite Conversion of thermally treated Y zeolite to the predominantly potassium exchanged form produces a unit cell size increase of approximately 0.02 to 0.04 A.
• the preferred unit cell size range of thermally treated Y zeolite is about 24.58 to 24.66 Angstroms
• the preferred range after potassium exchange is about 24.60 to 24.70 Angstroms.
• the feed stream for the separation process of the instant invention may be obtained from any of several sources.
• a mixed C aromatic feed stream containing the various xylene isomers and ethylbenzene may result from the steam cracking of hydrocarbon oils, from coal tar sources, or from the reforming or hydrocarbon oils in petroleum process operations.
• a C aromatic fraction is concentrated prior to separation, by distillation or other suitable means.
• the C aromatic product or feed stream to an aromatic isomerization process may be used as feed for the instant process.
• Any combination of paraxylene and the other xylene isomers and/or ethylbenzene would provide a suitable feed stream for the instant invention.
• the feed stream is passed over the predominantly potassium exchanged modified type Y zeolite in either the liquid or vapor phase.
• an inert carrying agent may be passed over the zeolite in conjunction with or immediately after the feed stream.
• Inert carriers which may be utilized include the following: Aromatics which are less tightly adsorbed than the xylenes, paraffins (both normal and iso-), olefins, naphthenes, and mixtures of the above.
• the basic requirements for an inert carrier are: (l) a liquid at adsorption conditions which does not react with the xylenes or zeolite at adsorption or desorption conditions; (2) which can easily be separated from the xylene product. Examples are paraffins from C to C (iso and normal), olefins from C to C (normal and branched), benzene, Tetralin, Decalin, and halogenated derivatives of the above.
• the preferred diluents for use as wash agents are low molecular weight paraffms, and olefins. These are effective wash agents for unadsorbed xylene removal and can be easily separated from the paraxylene product, e.g., pentane, isopentane, hexane, pentene, etc. In the absence of the diluent wash a vapor phase introduction of the feed stream is preferred.
• the diluent if incorporated with the feed would compose about 10 to 90 wt. percent of the feed stream, preferably 30 to wt. percent and most preferably 40 to 60 wt. percent.
• the presence of the diluent is important to successful operation in the liquid phase because the diluent serves to sweep the interstices of the zeolite adsorbent while the feed stream is passed over it.
• the preferred method of using liquid phase adsorption would be as follows:
• the mixed xylene and/or ethylbenzene feed stream would be fed over the adsorbent in the liquid phase, either undiluted or with an inert liquid carrier, until the desired effluent composition is obtained, this may be equilibrium where the effluent and feed stream have the same composition.
• An inert liquid carrier is passed over the adsorbent to wash the non-adsorbed xylene impurities from the interstices 3.
• the adsorbent is desorbed by steam or other means producing a desorbate which contains pure paraxylene and the inert carrier liquid.
• the paraxylene product and carrier liquid is separated by fractionation on or other conventional means.
• the product recovered is essentially paraxylene and contaminants such as ethylbenzene, meta-xylene or ortho-xylene are minimized since they have been swept from the interstices and are substantially absent from the adsorbent.
• the mixture is passed over the adsorbent in the vapor phase at a rate of 0.01 to 10 W/hr./W, preferably 0.1 to 5 W/hr./W.
• a feed rate of 0.1 to 5 W/hr./W may be utilized, preferably 0.2 to 2 W/hr./W.
• the adsorption technique of the instant invention be cyclic, but a continuous process is intended to be within the scope of the instant invention.
• Typical cycle times will vary between 2 and 480 minutes preferably 10 240 minutes and most preferably 15 120 minutes. Temperatures during adsorption may vary widely in the vapor phase. Acceptable limits would vary from 200 800 F, preferably 200 700 F, and most preferably 300 600 F. In the liquid phase the temperature may vary between 30 and v 300 F. while preferred temperatures are between 50 and 200 F. and most preferably 70 and 150 F. The pressure may vary widely; acceptable limits are between 0.1 and 700 psig, in both the liquid and vapor phases. For a vapor phase operation, a preferred pressure would be between 0.1 and 100 psig and most preferably between 10 and 50 psig. Pressure in the liquid phase should be sufficient to maintain liquid phase at the temperature utilized.
• a typical mixture which would be treated by the process of the instant invention would have about 10 to 30 wt. percent of para-xylene, 30 to 50 wt. percent of meta-xylene, 10 to 30 wt. percent of ortho-xylene and 10 to 30 wt. percent of ethylbenzene.
• the preferred separation of the instant invention is that of paraxylene from at least one of its isomers and/or ethylbenzene
• the instant invention may be used to obtain any one of the C aromatics used in pure form by proper use of adsorption and desorption techniques and stages.
• adsorption and desorption techniques and stages As mentioned, previously, there is a definite order in which the various xylene isomers and ethylbenzene will be attracted to a predominantly potassium exchange modified type Y zeolite.
• the zeolite must, as mentioned previously, be modified so that the unit cell size after potassium exchange is between 24.50 to 24.75 Angstroms and preferably 24.60 to 24.70 Angstroms. While the most effective separation utilizing the instant sieve is that of ethylbenzene and paraxylene, the separation of paraxylene and meta-xylene may also be performed effectively. Typical alphas for the separation of ethylbenzene and paraxylene are 3.5 to 4.5; alphas for the separation of ethylbenzene and metaxylene are 3.5 to 7.0; alphas for the separation of paraxylene and orthoxylene tend to be between 3.5 and 6.0.
• alpha is as follows: paraxylene Mole fraction other C Alpha. (EquiL)
• the expression denotes the alpha values measured are actually equilibrium alpha values. Experimentally, they are obtained by passing a feed mixture containing para-xylene, meta-xylene, ortho-xylene and ethylbenzene over a column of freshly-calcined adsorbent until effluent composition matched that of the feed, i.e., until the adsorbed and external phases were at equilibrium. If operating liquid phase, excess feed was then swept from the column with a n-paraffin wash of precisely 1.2 volumes per volume of adsorbent (14-35 mesh particles). The adsorbed phase was then desorbent with an appropriate agent, usually methyl alcohol. For vapor phase operation the excess feed was removed by purging the column with nitrogen and the adsorbed phase was then desorbed with methyl alcohol vapor or with steam. The feed and desorbate were analyzed by vapor phase chromatography.
• the para-xylene will have to be desorbed. Due to the stable nature of the thermally modified type Y zeolite, steam desorption may he used. Since it is readily available, and quite economical it is most preferred to use steam for desorbing paraxylene or any other compound adsorbed onto the predominantly potassium, modified type Y zeolite.
• the steam would be maintained at a temperature of 200-700 F., preferably 200-500 F. during desorption Pressure would vary between 0.1 and 300 psig, preferably 10 and 100.
• the steam is passed over the bed at a rate of 01 to 10 W/hr./W, preferably 0.5 to 2 W/hr./W
• Paraxylene recovered from the adsorbent after the desorption may be readily separated from the steam. Both para-xylene and steam are cooled to condense them into the liquid phase. The liquids are collected in a settling vessel and the water layer drawn off the bottom of the vessel. The hydrocarbon or para-xylene layer is removed from the top of the vessel and may be recovered as is or dried by treatment with an ap limbate drying agent such as alumina, silica gel of a 4A molecular sieve or other means which are well, known in the art.
• an ap limbate drying agent such as alumina, silica gel of a 4A molecular sieve or other means which are well, known in the art.
• desorption may be effected by passing a normal paraffin such as those generally in the range of C to about C ,i.e., normal pentane over the loaded bed at elevated temperatures.
• Aromatics may also be used to displace the para-xylene from the bed at elevated temperatures.
• Polar displacing agents such as S0 carbon dioxide, al-
• glycols such as ethylene glycol and propylene glycol
• halogenated compounds such as methyl chloride, ethyl chloride, methyl fluoride
• nitrogenated compounds such as nitromethane and the like
• the displacing agents may be passed over the adsorbent countercurrently or cocurrently; a countercurrent desorption cycle is preferred. Typical temperatures during desorption will vary between 30 and 700 F.,
• the method of measuring the unit cell size is as follows:
• Unit cell measurements were made using a Norelco X-ray diffractometer with a nickel-filter copper K radiation.
• the instrument was operated with 50 Kv operating potential and 20 ma.
• the sample to be run was mixed with about 10 percent of an alpha alumina internal standard and scanned from about 535 twotheta to about 585 two-theta at a goniometer speed of 025 per minute and a chart speed of one-half inch per minute.
• Example 1 This example illustrates the preparation and properties of four difierent versions of the modified, predominantly potassium exchanged Y zeolite adsorbents.
• Step B PREPARATION OF THE PREDOMINANTLY POTASSIUM EXCHANGED ADSORBENTS
• the various preparations of Step B were exchanged hydrothermally three times with a threefold molar excess (based on sodium content of the starting material) of potassium chloride. Each exchange was conducted for minutes at 120 F and the Y-zeolite was recovered by filtration prior to the second and third exchanges. Following the final exchange the zeolite particles were washed free of excess potassium chloride, dried, and formed into 14-35 mesh particles for adsorption studies.
• Example 2 This example contrasts the superior performance of the predominately potassium exchanged modified Y zeolite adsorbent with another potassium zeolite for the separation of para-xylene from ethylbenzene.
• Example 3 The variation of adsorbent characteristics with variation in the degree of thermal treatment used in their preparation is shown in Table V.
• the adsorbent preparations are those identified in Table III of Example 1. Adsorption tests were made at 78 F following procedure A of Example 2.
• a process for preferentially adsorbing a C aromatic from admixture with at least one other C aromatic which comprises contacting said mixture under adsorption conditions with a predominantly potassium type Y zeolite, said zeolite having a unit cell size of 24.50 to 24.75 Angstroms wherein at least a portion of said preferentially adsorbed C aromatic is adsorbed onto said zeolite, and desorbing said adsorbed aromatic from said zeolite whereby a stream enriched in said preferentially adsorbed aromatic is obtained.
• a process for preferentially adsorbing paraxylene from admixture with at least one compound selected from the group consisting of meta xylene, ortho xylene and ethyl benzene which comprises contacting said mixture under adsorption conditions with a type Y zeolite in which 70 to 98 percent of the sodium ions have been replaced by potassium ions, said zeolite having a unit cell size of 24.50 to 24.75 Angstroms wherein said paraxylene is preferentially adsorbed and desorbing said paraxylene from said zeolite.

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• 1970
  • 1970-10-01 US US77344A patent/US3686343A/en not_active Expired - Lifetime
• 1971
  • 1971-09-28 GB GB4504271A patent/GB1361652A/en not_active Expired
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